Black overprint
The Black overprint (for Pure Black) option specifies whether black text, or black text and vector graphics (where black is defined as RGB=0, 0, 0, or as CMYK=0%, 0%, 0%, 100%) print over colored backgrounds. If you turn off this option, black text, or black text and graphics knock out colored backgrounds. Knocking out can create a white edge around objects, or a white space, caused by the misalignment of color plates.
The Black overprint (for Pure Black) option has the following settings:
Text - Black text overprints colored backgrounds, eliminating white gaps and reducing the halo effect from misregistration of colors.
Text/Graphics - Black text and graphics overprint colored backgrounds, eliminating white gaps and the halo effect from misregistration of colors.
Off - Black text and graphics knock out colored backgrounds.
An example of how you might use this setting is a page that contains black text on a light blue background. The background blue is CMYK=40%, 30%, 0%, 0%. The black text is CMYK=0%, 0%, 0%, 100%.
With Black overprint (for Pure Black) set to Text or Text/Graphics, the final text or graphic portions of the page are overprinted, or combined with the underlying color. Black colors generated by applications (for example, RGB=0, 0, 0 or CMYK=0%, 0%, 0%, 100%) are printed using black only. This means that black text and line art do not exhibit halftone artifacts (as long as the printer is calibrated correctly). No transition in cyan and magenta occurs and the quality of the output is improved, because it does not show artifacts near the edges of text.
With Black overprint (for Pure Black) set to Off, the border of the text or graphic is on an edge that has cyan and magenta on one side (outside the object) and black on the other side (inside the object). This transition may cause visible artifacts due to the practical limitations of the printer.